1. Field of the Invention
The present invention relates to a novel combination of an interrupter switch and a distribution transformer and, more particularly, the combination of a gas-filled interrupter switch and an oil-filled distribution transformer, the switch being for controlling power to the transformer.
2. Description of the Related Art
When interrupting current to an inductive load, an arc forms between the separating switch contacts. For many years, the practice in the electrical power distribution industry has been the use of loadbreak switching in which contact arcing during current interruption occurs in an insulating fluid. The fluids historically in use, such as mineral and silicone oils, are limited in their arc-quenching capabilities and so are best suited to strictly insulating and cooling purposes. The existence and process of extinguishing the arc causes a breakdown of the insulating medium, general contamination from by-products of the fluid, and gas generation which raises the system pressure and may require venting of the enclosure. However, liquids are advantageous because of their low cost, ready availability and ease of handling and storage.
Gas insulation fluids have also been employed, for example, sulfur hexafluoride or SF.sub.6, because it has certain qualities which are superior to air or oil. Gas insulation is disadvantageous, however, because of its costs and difficulty in handling and containment.
SF.sub.6 is denser than air, non-flammable, chemically stable and inert at moderate temperatures. The electrical, thermal and chemical properties of SF.sub.6 make it an effective insulator that is especially superior to air and comparable to oil at moderate operating pressures. Interrupting electrical load current in an atmosphere of SF.sub.6 permits a greater energy interruption rating than is available with air in the same space. SF.sub.6 is superior to oil in arc-quenching and arc-cooling capabilities. SF.sub.6 at moderate pressure, for example, 2 atmospheres, provides a uniform insulative and thermally stable environment at all points within a container at equilibrium.
When electrical contacts are separated during loadbreak, SF.sub.6 flows readily into the gap to insulate the gap as does any fluid. However, SF.sub.6 has an additional advantage in that it is readily compressible. Thus, SF.sub.6 can be forced to flow into the arc path, through one or more well known pressurization methods so that arc current is interrupted quickly and more efficiently than with common liquid insulators.
Unlike oil, by-products resulting from arc dissociation of the SF.sub.6 gas tend to recombine into the gas after a short period of cooling, leaving little or no harmful residues in the system, causing minimal loss of insulation, and producing no requirement for venting. In addition, if extinguishing of the arc is delayed, the non-flammable character of SF.sub.6 reduces any fire hazard.
Although SF.sub.6 insulation has all the above-mentioned advantages, SF.sub.6 -insulated modules might sometimes leak, resulting in degraded operating characteristics and raising doubts as to the safety of the equipment. Leakage from large equipment enclosures for SF.sub.6 equipment seems to be so common as to be a recognized liability during consideration of the acquisition of SF.sub.6 equipment. Moreover, leaks are hard to detect because SF.sub.6 is colorless, odorless, tasteless, and leaves no residue. Only pressure monitoring instrumentation provides clear evidence of leakage. Small encapsulated modules, however, such as self-contained switches, can be sealed well enough so that any leakage is rare, except in the case of damage. Damage which occurs prior to installation is usually obvious to the assembler, and therefore is a minimal problem.
As a static insulation fluid, SF.sub.6 can be used satisfactorily in transformers. Although SF.sub.6 has good thermal characteristics, it's suitability for use in distribution transformers for cooling, however, is impeded by the poor natural convection flow of the gas. Transformer coils insulated with SF.sub.6 must be designed with an open structure and large ducts to allow easy passage of the gas, as in air-insulated, dry-type transformers, and may require forced circulation. This is a much less efficient use of space than immersion of the coils in liquid which is able to cool better due to the superior flow characteristics of liquids.